An earthquake swarm near Bombay Beach in the Salton Sea area began on March 21st, 2009, with a M1.8 earthquake, located 3km SSE of Bombay Beach. The swarm has continued through today, with the largest event
having occurred
yesterday morning at 04:55AM PDT, a M4.8 earthquake, located 4km S of Bombay Beach. So far, there have been more than 250 events between M0.8 and M4.8, and the events continue through 3/25/09, despite a five-hour lull in activity from 06:47PDT through 11:41PDT on 3/25/09.

The M4.8 moment tensor solution displays strike-slip motion, with the preferred nodal plane strike of N57E, with an uncertainty of 8 degrees. This is consistent with a fault or faults
orthogonal
to the San Andreas fault, located in the northern Brawley Seismic Zone. As time has progressed, the events have migrated slightly southeast. Before 2001, there were few events that had occurred in the northern Brawley Seismic Zone. The southern Brawley Seismic Zone is much more active, with events occurring on a regular basis.

From 1932 to 2008, we recorded 722 events within a 5km radius of this current swarm. Most of the historical events recorded within 5 km of the current swarm were portions of earlier Bombay Beach swarms. Historically, the larger events (>M3.0) occurred less frequently, but still total 22. Prior to this sequence, the largest event recorded in this 5km region of Bombay Beach was a M4.2 earthquake. This event occurred on 11/13/99, and was triggered by the Hector Mine Earthquake, which occurred on 10/16/1999. The list of current events for the swarm can be viewed below. We will continue to monitor the area, and post any new information here at SCSN.org.

The Hauksson-Shearer relocations of historical events (in white) display a ladder-type structure in the plan view, with the current sequence at the northwesternmost portion of the zone. These southwest to northeast trending events may be an extension of the Extra fault zone (shown in orange), but is more commonly referred to as the Brawley Seismic Zone.

The latest events are shown as red circles (precisely re-located as of 3/24/09 noon local time by Egill Hauksson of Caltech). The activity occurred near the intersection of the San Andreas fault and the northeastward projection of the Extra fault zone, near the northern end of the Brawley Seismic Zone. This area has been seismically active previously in Oct. 1999 (Hough and Kanamori, 2002) and Nov. 2001. It has also been noted as being potentially significant for future rupture of the southern San Andreas fault (e.g., Hudnut et al., 1989). Active faults are shown in red if they have ruptured historically (the Elmore Ranch fault zone broke in Nov. 1987, and the Coyote Creek fault broke in 1968). Also shown in orange are faults that have been mapped and that have exhibited triggered creep during regional earthquakes. These include the San Andreas fault and Extra fault zone [data of Susanne U. Janecke, unpublished mapping, Kirby 2005 MS thesis, and Kirby et al. (2007). Additional faults are being mapped.] Other features shown are volcanic centers (orange circles) and various mud pots, carbon dioxide vents (green symbols; from Lynch & Hudnut, 2008).

The image above shows the events which have been re-located using the Hauksson/Shearer method. Note that the events create a linear feature orthogonal to the San Andreas fault.

The following table lists all events M3 and larger that have occurred during this swarm:

#EVID

MAG

YYYY/MM/DD

HH:mm:SS

LAT

LON

Depth

Location

14432456

3.1

3/21/2009

13:12:18

33

18.8

N

115

43.8

W

4.7

4

km

(

3

mi)

S of

Bombay Beach, CA

14432496

3.3

3/21/2009

13:17:03

33

18.9

N

115

44

W

4.7

4

km

(

2

mi)

S of

Bombay Beach, CA

14432576

3.1

3/21/2009

13:40:06

33

18.9

N

115

44

W

4.9

4

km

(

2

mi)

S of

Bombay Beach, CA

14433456

4.8

3/24/2009

4:55:43

33

19.1

N

115

43.7

W

5.8

4

km

(

2

mi)

S of

Bombay Beach, CA

14433696

3.1

3/24/2009

6:52:51

33

18.8

N

115

44.2

W

4.2

4

km

(

2

mi)

S of

Bombay Beach, CA

14434264

3.1

3/24/2009

16:43:15

33

18.4

N

115

44.6

W

5

5

km

(

3

mi)

S of

Bombay Beach, CA

14434504

3

3/24/2009

22:50:55

33

17.4

N

115

43.5

W

6

7

km

(

4

mi)

S of

Bombay Beach, CA

14434688

3.6

3/25/2009

0:51:23

33

17.4

N

115

43.3

W

4.4

7

km

(

4

mi)

S of

Bombay Beach, CA

14435296

3.7

3/25/2009

12:59:44

33

17.5

N

115

43.2

W

6.5

7

km

(

4

mi)

S of

Bombay Beach, CA

14435776

4

3/25/2009

20:25:21

33

17.6

N

115

43.3

W

7.4

6

km

(

4

mi)

S of

Bombay Beach, CA

10111842

3.4

3/25/2009

20:25:50

33

18.1

N

115

42.4

W

5

6

km

(

4

mi)

SSE

of Bombay Beach,

CA

14436024

3

3/25/2009

22:12:33

33

17.5

N

115

43.1

W

11

7

km

(

4

mi)

SSE

of Bombay Beach,

CA

The image below shows the relocations plotted using the Hauksson-Shearer method:

The graphs above represent the swarm and are color-coded by magnitude: